The treatment of inherited metabolic liver diseases by hepatocyte transplantation (HT) would be greatly facilitated if the transplanted normal hepatocytes could be induced to proliferate preferentially over the host liver cells. We hypothesized that preparative hepatic irradiation (HIR) should inhibit host hepatocyte proliferation in response to partial hepatectomy (PH). Normal nonirradiated hepatocytes transplanted in this setting should have a selective growth advantage over the host liver cells and should progressively repopulate the liver. To test this hypothesis, we transplanted 5 million hepatocytes from normal Wistar-Roman High Avoidance (RHA) rats into the livers of congeneic bilirubin-uridine 5'-diphosphoglucuronate glucuronosyltransferase (UGT1A1)-deficient jaundiced Gunn rats by intrasplenic injection after one of the following treatments: (1) 68% PH, (2) HIR (50 Gy), or (3) HIR + PH. In rats receiving either PH or HIR alone before HT, serum bilirubin concentrations declined by 25% to 30% in 28 weeks. In contrast, serum bilirubin levels were normalized completely in rats receiving HIR + PH before HT. Massive repopulation of the Gunn rat liver by the UGT1A1-positive Wistar-RHA hepatocytes was shown by UGT1A1 enzyme assay, immunoblot analysis, and immunohistochemical staining of the recipient liver. High-performance liquid chromatography analysis of the bile collected from Gunn rats 5 months after PH, HIR, and HT showed normalization of the pigment profile, with bilirubin diglucuronide and monoglucuronide as the predominant pigments. In conclusion, a preparative regimen of HIR + PH results in massive repopulation of the liver with functionally normal transplanted hepatocytes, resulting in complete correction of a metabolic deficiency. Noninvasive strategies to replace PH for providing proliferative stimuli to the transplanted cells should make this regimen valuable in augmenting the effects of HT for the treatment of liver diseases.